[PyTorch] Use new cuda semantics

[tabergma]

As described in the CUDA semantics by PyTorch we should use

device = None
if torch.cuda.is_available():
    device = torch.device('cuda')
else:
    device = torch.device('cpu')

x = torch.empty((8, 42), device=device)
net = Network().to(device=device)

instead of

x = torch.empty((8,42))
if torch.cuda.is_available():
    x = x.cuda()